Microwave or millimeter wave communication systems, which are line-of-sight communication systems with fixed transmitting and receiving stations, have recently been growing rapidly in demand as a mobile communications infrastructure. With the increasing traffic of the mobile communications, a further increase in the transmission capacity has been demanded.
According to such a market demand, dual polarization transmission with XPICs (Cross Polarization Interference Cancellers), which have conventionally been used in trunk systems, have come into standard use. A typical high-capacity system at present is configured to use 128 QAM (Quadrature Amplitude Modulation) with a bandwidth of 28 MHz to transmit two STM (Synchronous Transfer Mode)-1 signals. To increase the transmission capacity beyond such a system, the number of modulation levels can be made greater than 128 and the bandwidth wider than 28 MHz.
Increasing the number of modulation levels, however, has only a limited effect of capacity increase relative to an increase of what is called C/N (Carrier to Noise power ratio), resulting in the disadvantage of lower cost efficiency. Widening the bandwidth reduces the system gain as much as the noise bandwidth, with the disadvantage of increased device power consumption. Wider bandwidths (such as 56 MHz) may be unavailable in some countries. High frequency bands of 60 GHz and above can be used to provide a wide band, whereas too high frequencies increase the parts cost and have the disadvantage of an extremely short transmission range due to radio wave absorption by the air.
Under the foregoing limitations, it has been difficult to further increase microwave or millimeter wave communication systems in capacity.
As mentioned above, in a microwave or millimeter wave communication system that has fixed transmitting and receiving stations and uses a band of several to several tens of gigahertzs, the dual polarization transmission method of using two polarizations of radio waves to transmit two independent signals in the same frequency band is employed as a means for increasing the transmission capacity. Since the transmission quality deteriorates due to cross polarization interference, there are implemented XPICs for canceling the interference. The XPICs make reference to signals of the other polarizations to extract interference components leaking in their own polarizations, and subtract the interference components from the signals of their own polarizations for interference cancellation (for example, see NPL 1). The dual polarization transmission method with XPICs doubles the transmission capacity for an identical bandwidth. To achieve a further increase in capacity, however, there has been no means other than to increase the bandwidth.
By the way, mobile communications and wireless LAN systems, which are non-line-of-sight communication systems, have been increased in transmission capacity by MIMO (Multiple-Input Multiple-Output) technologies which use a plurality of antennas for transmission and reception. A plurality of signals are received on the receiving side because of ambient reflection. The state of reflection varies with time depending on the movement of the communication device itself or surrounding objects that are causing scattering. The use of a plurality of antennas for transmission and reception is equivalent to a situation where there are a plurality of transmission channels, so that independent signals corresponding to the number of antennas can be transmitted in the same frequency band. Nevertheless, there is no mobile communications system that uses dual polarization.
It has heretofore been held that the same MIMO technologies as those of mobile systems are not applicable to microwave or millimeter wave communication systems for line-of-sight communications. This is due to the following reason. The transmission-reception distance (for example, several to several tens of kilometers) is extremely large as compared to a feasible spacing for antenna installation (such as several meters), and in the absence of constant scattering environment, the signals transmitted from respective antennas have an extremely high correlation with each other. As a result, the transmission channels appear to be only one, and the number of signals that can be transmitted in the same frequency band becomes one. That is, in line-of-sight communication systems, MIMO-based parallel communication channels such as in practical use with mobile units do not hold.
In a certain condition, however, it is possible to form a plurality of independent communication channels even in a line-of-sight scatterless environment. The principle is described on pp. 174-5 of NPL 2, a conventional art document relevant to this application, as follows. With widely spaced antennas, signals that arrive at a single receiving antenna from a plurality of transmitting antennas have almost the same transmission distances and can thus be considered to have the same amplitude. The slight differences in the transmission distance, however, result in different phases due to the high frequency. The presence of such a phase difference makes the rank of the communication channel matrix H (the elements of H are the transfer functions between the antennas, rendered in a complex representation of amplitude and phase) equal to the order of the matrix H. That is, there are as many independent communication channels as the number of antennas. For example, with arrays of two antennas, the matrix H is a 2-by-2 square matrix having a rank of 2, so that there are formed two communication channels. When using arrays of two antennas for transmission and reception, respectively, the communication channel matrix H is given by the following:
                    H        =                  (                                                                      h                  11                                                                              h                  12                                                                                                      h                  21                                                                              h                  22                                                              )                                    [                  Eq          .                                          ⁢          1                ]            where hij is the complex transfer function (phase term*amplitude term) from a transmitting antenna j to a receiving antenna i (the latter suffix indicates the transmitting side). The rank of the communication channel matrix H is the number of independent communication channels.
What is important here is that the formation of communication channels based on such a principle is a fixed one under geometric conditions, not stochastically variable as with ordinary MIMO. As an example, description will be given of the configuration of FIG. 1. With a transmission range of R and an RF frequency of f1, the antenna installation spacing is set so that signals transmitted from the two antennas on the transmitting side (Tx) are received by a single antenna on the receiving side (Rx) with a phase difference (in the diagram, the difference between R1 and R3 and the difference between R2 and R4) of λ/4 (λ is one wavelength). The signals A and B to be transmitted from the two antennas are given a difference of π/2 in phase. Consequently, the signal A is enhanced and the signal B is cancelled out at either one of the receiving antennas, and vice versa at the other antenna. This makes it possible to separate the two signals transmitted at the same frequency into two on the receiving side. Such signal separation does not need a scattering environment for the communication channels unlike ordinary stochastic MIMO.
The principle of the MIMO system described in NPL 2, however, holds only in very special conditions. The principle has thus been impractical since such conditions easily fail to hold because of an error in the actual installation spacing between the antennas or due to small swings of the antennas in the wind or by vibration.
As a means for solving the problem, unpublished PTL 1 which the applicant has filed as relevant technology prior to this application describes a MIMO system that can apply SVD (Singular Value Decomposition), which is used in mobile MIMO, and other matrix operations to form stable communication channels despite the presence of the foregoing variable factors.
In other words, the MIMO signal processing technology described in unpublished PTL 1 can be applied to the principle configuration shown in NPL 2 to increase the transmission capacity of even a line-of-sight communication system through stable spatial multiplexing.